1. Field of the Invention
The invention relates to a fuel injector, in particular a common rail injector, for injecting fuel into a combustion chamber of an internal combustion engine.
2. Description of the Prior Art
The highest priority in the development of internal combustion engines is devoted to adhering to pollutant limit values. Precisely the common rail injection system has made a decisive contribution to reducing pollutants. The advantage of common rail systems is their independence of the injection pressure on the rpm and load. For meeting future exhaust gas limit values, however, a significant increase in the injection pressure is necessary precisely with Diesel engines.
Stroke-controlled common rail injectors are known whose injection valve element is servo-operated. Piezoelectric and magnet valves are used as pressure adjusters and with them the servo circuit is controlled. For fast needle closure, a permanent low-pressure stage is often provided, which exerts a permanent closing hydraulic force on the needle. The disadvantage is the high amount of leakage that ensues between the high-pressure and the low-pressure stage. Leakage unavoidably leads to the necessity of higher pumping power and thus to sacrifices in system efficiency. This situation becomes especially problematic at high pressures. For that reason, the latest injectors are designed to be leak-free at extremely high injection pressures.
In contrast to conventional designs, these so-called leak-free fuel injectors have no permanent low-pressure stage acting in the closing direction, and as a result the attendant leakage points are eliminated. Because of the eliminated low-pressure stage, two-part injection valve elements of the kind used in the fuel injectors with a low-pressure stage that are used in the industry are no longer employed.
While in modern mass-production injectors with a low-pressure stage, both injection valve element parts (control rod and nozzle needle) are pressed against one another because of the resultant pressure forces, in the case of leak-free fuel injectors a separate form- or force-locking connection must be established. For coupling to injection valve elements, it has become known to provide a hydraulic coupler volume between them. The coupler volume is typically realized in the form of a coupler sleeve in which one of the injection valve element parts is guided. The coupler volume is reduced by expelling fuel through the guide gap between the injection valve element part guided in the sleeve and the sleeve itself.